In a telephony data access arrangement (DAA) circuit, there is a need to monitor (snoop) the line when the system is in an on-hook condition. In a transformer based DAA, this has to be done without the help of any direct DC monitoring, or communication, as the transformer isolates these components. There are techniques in use that take advantage of an extra monitor loop that bypasses the transformer by use of an extra and expensive opto-isolation device.
Typically, a conventional modem ring detect circuit uses an opto-isolator, two zener diodes, three resistors and a large film capacitor, e.g. typically 0.47 uF, 250 V. Conventional circuits typically produce a single pulse per ring cycle (13 Hz-68 Hz) output for ring signals above a set threshold (typically around 25 VRMS). The opto-isolator and the large film capacitor are relatively expensive compared to the other components and the opto-isolator must be laid out on the circuit board in such a way as to maintain regulatory voltage isolation. The latter requires board space and creates other layout constraints. Another problem with such a conventional circuit is that the ring detect threshold is determined by the zener diode component values and, as a consequence, it is not as easy to change the threshold when using these components as it is when using resistors.
In a basic data access arrangement (DAA), there are two other main functions, besides ring detection, that need voltage isolation—hook control and the audio signal paths. One method for reducing the cost of the DAA is to integrate or encode the ringer information on the audio signal path. One method of doing this in a transformer DAA is to use the on hook audio snoop path, which is commonly used for Caller ID, that goes around the hook switch.
To build an analog low frequency filter and detect circuitry generally requires components external to an integrated circuit (IC) or analog to digital (A/D) conversion with significant dedicated digital signal processing; either of these approaches increases costs.